The Russian energy giant Gazprom has promised to start pumping gas to Europe from this morning, a week after the pipelines from Siberia went empty, plunging half the continent into a winter fuel crisis.

Alexander Medvedev, deputy chief executive of the state gas monopoly, indicated yesterday that the final hurdles in the two-week dispute with Ukraine had been removed in a deal brokered by the EU. But Igor Sechin, Russia's deputy prime minister and head of the energy sector, told the Guardian the resumption of supplies from 8am would be "only a test delivery".

Most of the continent's gas imports from Russia arrive through the pipelines in Ukraine. The end to the dispute depends on EU monitors being allowed to verify the flow of gas in Russia, Ukraine, and Europe. Russian and Ukrainian experts are to join the European monitors.

While the EU monitors were in position last night, Sechin said the Russian experts were not yet stationed in Ukraine, signalling that the deal could still be aborted. Differences over who should pay for "fuel" gas that is used to keep up the pressure in the pipelines for pumping the natural gas to clients could also disrupt the fragile accord and spell more misery for hundreds of thousands of households in central and southern Europe.

Slovakia, which is dependent on Russian gas, said it would have to restart its decommissioned reactor at Bohunice, near the border with Austria, because it had received no supplies since Thursday. The nuclear plant is widely viewed as unsafe and its closure was one of the conditions for Slovakia joining the EU in 2004.

Austria, which has banned nuclear power, deplored the move. The environment minister, Nikolaus Berlakovich, described decision to restart the Soviet-era reactor as "completely unacceptable".

The European commission said any recommissioning of Bohunice would be a breach of European law and Bratislava's membership terms.

• This article was amended on Thursday 15 January 2009. The Bohunice nuclear power station in Slovakia does not have a Chernobyl-design reactor. Its four reactors are of a different design. This has been corrected.